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King County Water Quality Monitoring

King County has been conducting monthly baseline water quality monitoring in Tibbetts Creek since 1975. The A630 site was located near the Sammamish State Park entrance and sampled from 1975 to 1997. This site was moved when the parking lot near Tibbetts Creek was revamped. The X630 site is located at the footbridge crossing over Tibbetts Creek. Sampling at X630 was discontinued in 2008 due to budget cutbacks, but was reinstated in February 2013. The sites are not comparable due to a small tributary entering upstream of A630 and downstream of X630. King County began collecting benthic macroinvertebrate samples in Tibbetts Creek in 2002. Sediment samples have been collected from Tibbetts Creek as part of the Streams Sediment Monitoring Program starting in 1987.


Water Shed Image

Tibbetts Creek is a relatively short creek (4.3 miles long) that originates on Squak Mountain at an elevation of 1,080 feet. The stream flows through a steep headwater area, then in midcourse opens up to a broad flood plain that empties into Lake Sammamish. Though not a tributary to Issaquah Creek, it shares a common floodplain with the mainstem during large flood events.

Efforts to restore the damaged creek have been underway for more than ten years. In the summer of 2002, more than 10,000 cubic yards of mine tailings were excavated from where the debris had been dumped into a gully along Tibbetts Creek by mine operators. Prior to excavation, heavy rains would wash large quantities of the tailings into the creek. The sediment clogged channels, which impeded salmon and trout migration and contributed to downstream flooding. The mine closed in the 1950s (Seattle Times, November 22, 2002).

A site, roughly 5-acres in size, was replanted in 2002 on part of a 400-acre parcel between Cougar and Squak mountains deeded to Issaquah in May by the Talus developers. The transfer preserves valuable wildlife habitat, creating a mile-wide swath of public land that connects the Cougar Mountain and Squak Mountain parks. The restoration is a joint effort by Issaquah, local environmental groups, and the developers of Talus. Its purpose is to restore fish habitat, reduce flooding, and eliminate a significant source of phosphorus

About 1,800 feet of new stream channel, complete with standing snags and strategically placed logs, has been built on Rowley Enterprises property south of Interstate-90. Rowley Enterprises has committed $1.2 million to the restoration project. The process also included the Muckleshoot Tribe; the City of Issaquah; the volunteer Friends of the Issaquah Fish Hatchery, and the State Departments of Transportation, Parks, and Fish and Wildlife.

Total land use in the basin dominated by forest (mostly mixed forest) and developed land. Agriculture and other (barren and grassland) make up a very small portion of total land use (1% combined). Scrub and wetlands also comprise a small, albeit slightly larger, percent of land use. See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Agriculture Developed Forest Scrub Wetlands Other
Total <1% 30% 67% 2% <1% <1%


According to King County's 1989 - 1990 Status Report, Tibbetts Creek contains small populations of coho and cutthroat trout (Metro, 1990). No salmonids were observed in the creek by the volunteer Salmon Watcher Program in 2001 (King County, June 2002).

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen (DO), pH, conductivity, turbidity, total suspended solids, ortho-phosphorus, total phosphorus, ammonia, nitrate-nitrogen, total nitrogen, and fecal coliform (FC) bacteria. Results are compared to state water quality standards. Water quality standards are designed to protect public health and aquatic life. Comparing monitoring results to water quality standards allows an understanding of how safe the creek is for recreational contact as well as for aquatic life (see link at top of page to view current water data).

State water quality standards were revised in 2003. Tibbetts Creek is now categorized as “Core Summer Salmonid Habitat” for aquatic life use and “Extraordinary Contact” for recreational use. As part of the updated water quality standards, Tibbetts Creek has been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 16 ºC. Tibbetts Creek is on Ecology’s 303(d) list for violation of water temperature, DO, and bioassessment standards (Category 5). There are EPA-approved Issaquah Creek Basin Bacteria total maximum daily load (TMDL) plans in place and implemented to address FC bacteria (Category 4a).

See Table 2 below for routine monitoring summary statistics of water quality data collected to date. If stormwater data is available for this site, it will be shown as Table 3. Historical data reviews can be found in the annual reports produced by METRO/King County DNRP.

To view charts of current water quality data, please visit the Data Download webpage.

Water Quality Index

A Water Quality Index (WQI) rating system was developed by Ecology that evaluates several water quality parameters and gives a single rating of “high,” “moderate,” or “low” water quality concern. To see how these ratings compare with other stream sites, please visit the Water Quality Index webpage.

Table 1. Routine monitoring summary statistics for this station from 1997 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)2809.71.514.510.02.0
Temperature (°C)28211.
Turbidity (NTU)2817.381.30107.004.649.89
Conductivity (mSIEMS/cm)260222.679.0392.0196.080.6
Total Suspended Solids (mg/L)2818.310.50261.003.2020.28
Ortho-Phosphorus (mg/L)2810.01630.00420.08470.01400.0081
Total Phosphorus (mg/L)2810.05080.01470.27500.04440.0266
Ammonia (mg/L)2810.07880.01900.26800.06880.0438
Nitrate (mg/L)2810.95540.02803.55000.77600.7185
Total Nitrogen (mg/L)2811.23740.28603.67001.06000.6887
Fecal Coliform(CFU/100ML)24526541200082903

Table 2. Storm water monitoring summary statistics for this station from 1997 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)339.55.413.59.51.8
Temperature (°C)3411.15.818.211.03.5
Turbidity (NTU)3458.723.97894.0019.30156.04
Conductivity (mSIEMS/cm)28122.258.0209.0108.543.0
Total Suspended Solids (mg/L)3485.831.901380.0023.65239.36
Ortho-Phosphorus (mg/L)340.01910.00830.13600.01520.0210
Total Phosphorus (mg/L)340.12070.03331.16000.08360.1894
Ammonia (mg/L)340.05790.01100.19800.04410.0384
Nitrate (mg/L)341.16810.20303.19000.79450.9350
Total Nitrogen (mg/L)341.75490.55006.83001.34001.2504
Fecal Coliform(CFU/100ML)341841701600011002891


King County operates one precipitation gage on Tibbetts Creek: Tibbetts Creek Rain Gage (67u).

Stream Sediment

Benthic Invertebrates

Benthic macroinvertebrates are small animals visible to the naked eye (macro) that lack a backbone (invertebrate) and live in or around the streambed (benthic). This group includes aquatic insects (such as mayflies and dragonflies), crustaceans, clams, snails, and worms. Benthic macroinvertebrates are of interest to scientists and water resource managers because they are an excellent indicator of the biological health of stream ecosystems and are a critical component of the food web in aquatic communities. Scientists quantify the composition and diversity of benthic macroinvertebrate populations in a stream to compare the biologic integrity of different streams. King County has utilized benthic macroinvertebrate sampling to assess biological health of numerous creeks across the county (see map).

For more information about benthic macroinvertebrates and King County’s Benthic Macroinvertebrate Program, please visit the King County Stream Bug Monitoring webpage. To see the benthic macroinvertebrate data for Tibbetts, please visit the Puget Sound Stream Benthos webpage.

Special Studies